Lumenal prostheses are used for a variety of medical purposes. Stents, for example, can be placed in various body lumens, such as a blood vessel, biliary tract and gastrointestinal tract, for maintaining patency.
Stents placed in a blood vessel or a coronary artery are typically used to maintain the opening of the vessel wall. In order to do this, the stent must be deployed to the site within the vessel where support is needed, which can be accomplished in a variety of ways. In one common procedure, referred to as percutaneous transluminal coronary angioplasty (PTCA), a balloon catheter is used to open the vessel walls for creating a passageway. A stent is then placed in the newly opened passageway to prop open the vessel. An alternative method of stenting is direct stenting. Direct stenting is a medical technique in which dilatation occurs following implantation of the stent. It is believed that direct stenting may lead to less injury and result in less recurrent in-stent restenosis.
The stent may be self-expanding or balloon-expandable. Self-expanding and balloon-expandable stents are positioned at the deployment site of a vessel via a stent delivery system, which typically includes a delivery catheter. The self-expanding stents are compressed around the outside of the delivery catheter. A retractactable sheath is typically utilized to deploy the self-expanding stent. The stent is then maneuvered through a tortuous path of the patient's vasculature to the deployment site. It is advantageous for the stent to have a small outside diameter or profile in order to be maneuvered through and deployed in a restricted area such as an atherosclerotic site of a blood vessel.
Stents are categorized by a so-called “ratio” that is a measure of the outside diameter of the stent in its expanded state compared to the outside diameter of the stent in its compressed state. Present day stents typically have an expanded to compressed stent diameter ratio up to about 5:1. This relatively small ratio between the expanded and compressed diameters restricts the applications of currently available stents. For example, a stent having a profile small enough to permit the stent to be maneuvered through small diameter vessels would not have a large enough expanded diameter to be deployed in large vessels.
Present day large diameter stents typically have larger strut lengths than small diameter stents. The compressed diameter of current large diameter stents is limited due to interference between adjacent struts. In addition, in self-expandable stents, interference between adjacent struts may subject a portion of a strut to high stress/strain concentrations which may prevent the stent from fully expanding when deployed. For example, if a self-expanding stent is compressed beyond its elastic limit in an attempt to provide a smaller outside diameter, the stent will not return to its desired deployed expanded diameter due to permanent deformation. In addition, large strut lengths decrease the flexibility of the stent, as well as the scaffolding and coverage of the vessel wall when deployed.
Therefore, a need exists for a stent that has a large ratio between expanded and compressed diameters. This increased ratio will allow the stent to be used in more clinical situations than a stent with a smaller ratio. In addition, a need exists for a stent with the capability of minimizing the compressed profile of the stent while achieving optimal strain distribution. In tightly compressed stents, the distribution of strains to more suitable parts of the stent is needed in order to provide a stent capable of expanding to its fully intended diameter. Finally, a need exists for a stent that optimizes strut length to increase the expanded or deployed stent diameter without the detrimental effects of increased strain or compressed profile.